For ultraviolet-shielding cosmetic materials, a metal oxide such as titanium oxide, zinc oxide or cerium oxide is widely used as an inorganic ultraviolet-shielding material having excellent ultraviolet-shielding capability and high safety.
However, if such a metal oxide is blended as it is in a cosmetic material, there arise problems such as uncomfortable feeling on use or adverse effect on the human body due to photocatalytic activity of the metal oxide. Accordingly, the metal oxide surface is covered with an inorganic substance having no photocatalytic activity. Metal oxides covered with alumina or silica are commercially available but a product satisfied in both the inhibition of photocatalytic activity by coating and good use feeling when blended in a cosmetic material, is not known.
The present inventors have previously disclosed a silica-coated metal oxide having a silica film where the ratio I of the absorption peak intensity at from 1,150 to 1,250 cm−1 to the absorption peak intensity at from 1,000 to 1,100 cm−1 on the infrared absorption spectrum (I=I1/I2, wherein I1 is a maximum absorption peak intensity in the range of 1,150 to 1,250 cm−1 and I2 is a maximum absorption peak intensity in the range of 1,000 to 1,100 cm−1) is 0.2 or more and the refractive index is 1.435 or more; a production process thereof; and a cosmetic material containing the silica-coated metal oxide. Also, it has been verified that an ultraviolet-shielding cosmetic material ensuring good use feeling, high effect of preventing photocatalytic activity and excellent storage stability can be obtained by containing the above-described silica-coated metal oxide where a silica film is coated to a film thickness of 0.1 to 100 nm and the photocatalytic activity determined by a tetralin auto-oxidation method is 60 Pa/min or less.
In recent years, the ultraviolet-shielding cosmetic material is required to have good use feeling or high transparency in addition to high ultraviolet-shielding capability. Accordingly, the metal oxide used as an ultraviolet-shielding material is also demanded to have a primary particle size smaller than conventional materials and have excellent dispersibility so as to give good use feeling or high transparency when blended in a cosmetic material. The above-described silica-coated metal oxide previously proposed by the present inventors has excellent properties such as inhibition of photocatalytic activity and good use feeling but in order to elevate the transparency on blending in a cosmetic material, production of finer particles and improvement of dispersibility are in need of improvement.
However, a metal oxide powder having a small primary particle size is difficult to highly disperse because if it is suspended in a solvent, aggregation takes place. Therefore, before the coating with silica or during the silica coating reaction, an extra step such as ultrasonic dispersion treatment or cracking treatment by wet beads mill, homogenizer or the like is necessary and this is a problem in view of profitability.